1. Field of the Invention
The present invention relates generally to mobility support in a mobile communication system, and in particular, to a method of cell selection in a user equipment (UE) supporting Multimedia Broadcast/Multicast Service (MBMS).
2. Description of the Related Art
Currently, as a result of the development of communication technology, services provided in a Code Division Multiple Access (CDMA) mobile communication system are developing into Multicasting/Multimedia Communication services for transmitting the existing voice service data and additional large amounts of data such as packet data and circuit data.
In a Universal Mobile Telecommunication Service (UMTS) system, which is a 3rd generation (3G) mobile communication system employing Wideband Code Division Multiple Access (WCDMA) based on Global System for Mobile communications (GSM) and General Packet Radio Services (GPRS), Broadcast/Multicast Service in which the same data stream is provided from one data source to a plurality of user equipments (UEs) is supported in order to support the Multicasting/Multimedia Communication. The Broadcast/Multicast Service can be classified into Cell Broadcast Service (CBS), which is a message-oriented service, and Multimedia Broadcast/Multicast Service (MBMS service) for supporting multimedia data including real-time images and voice, still images, and text.
FIG. 1 is a diagram schematically illustrating a network configuration for providing an MBMS service in a mobile communication system. Referring to FIG. 1, a multicast/broadcast-service center (MB-SC) 110 provides an MBMS stream, and the MS-SC 110 schedules an MBMS service stream and sends the scheduled MBMS service stream to a transit network (NW) 120. The transit network 120 is a network existing between the MB-SC 110 and a serving GPRS support node (SGSN) 130, which sends the MBMS service stream provided from the MB-SC 110 to the SGSN 130. The transit network 120 is comprised of a gateway GPRS support node (GGSN) and an external network.
The SGSN 130 receiving an MBMS service stream via the transit network 120 controls an MBMS service for the subscribers, i.e., UEs 161, 162, 163, 171, and 172, requesting the MBMS service. For example, the SGSN 130 manages MBMS service accounting data for each of the subscribers, and selectively transmits MBMS service data to an associated radio network controller (RNC) 140. In addition, the SGSN 130 generates and manages a service context for the MBMS service and sends a stream for the MBMS service to the RNC 140.
The RNC 140 transmits the MBMS service stream to Node Bs 160 and 170 controlling the cells where the UEs 161, 162, 163, 171, and 172 requesting an MBMS service are located, among the Node Bs managed by the RNC 140 itself The RNC 140 and the Node Bs 160 and 170 constitute a UMTS terrestrial radio access network (UTRAN).
A cell 1, which belongs to the Node B 160 that is requesting a particular MBMS service, includes the UE1 161, the UE2 162, and the UE3 163, and a cell 2, which belongs to the Node B 170, includes the UE4 171 and the UE5 172. Herein, the term “cell” has a meaning similar to that of the term “Node B.” The RNC 140 controls radio channels set up between the Node Bs 160 and 170 and the UEs 161, 162, 163, 171, and 172 in order to provide the MBMS service. As illustrated in FIG. 1, one radio channel is established between one Node B and UEs belonging to the Node B in order to provide an MBMS service.
In such an MBMS service system, when a UE receiving an MBMS service moves from one cell, i.e., a serving cell, to another cell, i.e., a target cell, the control of the UE must be handed over from the serving cell to the target cell by a cell reselection procedure. More specifically, the UE must acquire information on the target cell in order to continuously use the same MBMS service as that used in the serving cell. The target cell information includes control information for accessing a transport channel for carrying a data stream for an MBMS service in the target cell. As a result, during cell reselection, delay occurs when acquiring information on the target cell by the UE. Further, this delay prevents the user from seamlessly receiving the MBMS service while moving between cells. Accordingly, there is a demand for cell reselection technology that enables a UE to rapidly acquire information on a target cell while moving between cells, thereby seamlessly providing an MBMS service.